#' @param x,attrname either a square matrix of covariates, one for each possible
#'   edge in the network, the name of a network attribute of covariates,
#'   or a network; if the latter, optional argument `attrname` provides
#'   the name of the quantitative edge attribute to use for covariate values
#'   (in this case, missing edges in `x` are assigned a covariate value
#'   of zero).
